The present invention relates to specially oriented polycrystalline thin films of transition metal chalcogenides with layered structure and to methods for their fabrication and more particularly, to polycrystalline thin films of semiconducting transition metal dichalcogenides which are oriented so that the basal plane of the crystallites is substantially exclusively parallel to the substrate surface, and to methods for their fabrication.
There has been recent interest in the preparation of polycrystalline thin films of semiconducting transition metal dichalcogenides, e.g., WS.sub.2, WSe.sub.2, MoS.sub.2 and MoSe.sub.2. Possible applications include electrochemical and photovoltaic solar cells (see H. D. Abruna and A. J. Bard, J. Electrochem. Soc., 129 (1982) 673; G. Djemal et al., Sol. Energy Mater., 5 (1981) 403). Interest also exists in use of such as thin films in solid lubrication, such in high or low temperature environments or under ultra high vacuum where liquid lubricants are not suitable (see H. Dimigen et al., Thin Solid Films, 64 (1979) 221). Other possible applications involve battery cathodes (see J. Rouxel and R. A. Brec, Rev. Mater. Sci., 16 (1986) 137), and catalysis (see R. R. Chianelli, Catal. Rev. Sci. Eng., 26 (1984) 361). These thin films possess a layered-type structure with strong anisotropy of their mechanical and electrical properties. The chemical bonding is much stronger within the layers (covalent bonds) than between them (Van der Waals (VdW) bonds) (see J. A. Wilson and A. D. Yoffe, Adv. Phys., 18 (1969) 193). Polycrystalline thin films of transition metal chalcogenides ordinarily grow with a lamellar structure in which the basal (or VdW) planes are perpendicular to the substrate (see R. Bichsel and F. Levy, Thin Solid Films, 116 (1984) 367; P. A. Bertrand, J. Mater. Res., 4 (1989) 180). However, for photovoltaic as well as for solid lubrication applications, growth with the VdW planes parallel to the substrate is desired (see P. D. Fleischauer, Thin Solid Films, 154 (1987) 309).
Polycrystalline thin films of transition metal dichalcogenides have been reported to be prepared mainly by sputtering the transition metal dichalcogenides (see R. Bichsel and F. Levy, Thin Solid Films, 116 (1984) 367; A. Mallouky and J. C. Bernede, Thin Solid Films, 158 (1988) 285), by electrochemical deposition (see S. Chandra and S. Sahu, J. Phys. D., 17 (1984) 2115) and by soft selenization or sulfurization in a closed tube system as was recently reported by Jager-Waldau et al. for tungsten (see A. Jager-Waldau and E. Bucher, Thin Solid Films, 200 (1991) 157) and molybdenum (see A. Jager-Waldau et al., Thin Solid Films, 189 (1990) 339) diselenides and disulfides. The average crystallite size obtained was between 10 and 80 nm.
In the soft selenization method, thin sputtered tungsten or molybdenum films were reacted with selenium in a closed tube system for tens to hundreds of hours. As a result, two types of orientations were obtained, with the basal plane of the crystallites being oriented either predominantly perpendicular (hereinafter Type I) or predominantly parallel (hereinafter Type II) to the substrate surface. The films of Type II resulting from the soft selenization method also included several percent of Type I mixed in, and are never substantially exclusively Type II. Since photovoltaic cells require films which are virtually exclusively parallel oriented, these films could not be used successfully for photovoltaic cells.
Thin films of materials such as MoS.sub.2, MoSe.sub.2, MoTe.sub.2, WS.sub.2, WSe.sub.2, ZrS.sub.2, ZrSe.sub.2, HfS.sub.2, HfSe.sub.2, PtS.sub.2, ReS.sub.2, ReSe.sub.2, TiS.sub.3, ZrS.sub.3, ZrSe.sub.3, HfS.sub.3, HfSe.sub.3, TiS.sub.2, TaS.sub.2, TaSe.sub.2, NbS.sub.2, NbSe.sub.2, and NbTe.sub.2 have been studied for both their photovoltaic and tribological properties. It has been found in both cases that thin films with Van der Waals (VdW) planes parallel to the substrate (Type II) are much preferable to those with VdW planes perpendicular to the substrate (Type I). This is especially true for photovoltaic applications where Type I structures normally exhibit very poor photovoltaic activity and where even a small percentage of Type I orientation in a predominantly Type II structure can severely degrade photovoltaic performance. It is believed that Type I structures act as recombination sites and as centers for electrochemical corrosion (see H. J. Lewerenz et al., J. Am. Chem. Soc. 102 (1980) 1877).
"Large Grain Copper Indium Diselenide Films", T. L. Chu et al., J. Electrochem. Soc., Sept. 1984, p. 2182, refers to the preparation of CuInSe.sub.2 films in an open tube reactor in the presence of a forming gas (a mixture of 5% hydrogen and 95% nitrogen). Since this is an almost isotropic material, there is no reason to expect any preferred orientation for the CuInSe.sub.2 film. One skilled in the art would not have thought to use this method with an anisotropic material like WS.sub.2 or WSe.sub.2.
It has heretofore been unknown how to make polycrystalline thin films of transition metal chalcogenides which are substantially exclusively of Type II. Furthermore, it has been unknown how to make such polycrystalline thin films which are continuous films wherein the average crystallite size is very large (a few mm by a few mm).
There is thus a widely recognized need for, and it would be highly advantageous to have, polycrystalline thin films of semiconducting transition metal dichalcogenides, and methods for making these thin films, which will be oriented substantially exclusively as a Type II structure.